Abstract
Vaccinia-related kinase 1 (VRK1) is a pro-proliferative nuclear kinase. Mice engrafted with VRK1-depleted MDA-MB-231 breast cancer cells have been shown to develop fewer distal metastases than controls, suggesting VRK1 might play a role in cell migration, invasion, and/or colonization. In work described herein, we investigated the impact of VRK1 overexpression on human mammary epithelial cells. In 2D culture, VRK1 overexpression diminishes cell migration and invasion and impairs the migration-associated processes of cell spreading and cytoskeletal rearrangement. VRK1-overexpressing cells show reduced accumulation of the mesenchymal marker vimentin and increased accumulation of the epithelial markers E-cadherin and claudin-1. VRK1 overexpression also leads to reduced levels of the transcriptional repressors snail, slug, and twist1. Cumulatively, these data indicate that VRK1 overexpression augments the epithelial properties of both MCF10a and MDA-MB-231 cells. We further studied the impact of VRK1 on the epithelial properties of MCF10a cells in 3D matrigel culture, in which cells proliferate and form epithelial sheets that mature into hollow spherical acini. VRK1 overexpression significantly accelerates the initial stages of cell proliferation, leading to larger acini that nevertheless differentiate and mature. Our analysis of human tumor tissue microarrays (TMAs) revealed that VRK1 protein levels are higher in lymph node metastases than in patient-matched mammary tumors. Using public databases, we determined that VRK1 is among the top 10% of overexpressed transcripts in multiple subtypes of invasive breast cancer, and that high levels of VRK1 expression are correlated with decreased relapse-free survival. In sum, overexpression of VRK1, by regulating the transcription repressors snail, slug, and twist1, can promote a mesenchymal-to-epithelial transition (MET) in cell culture. VRK1-mediated MET might facilitate the colonization of distal sites by metastatic breast cancer cells, providing some insight into the frequent association of VRK1 overexpression with breast malignancies and the correlation between VRK1 overexpression and poor clinical outcome.
Highlights
Vaccinia-related kinase 1 (VRK1) is a serine/threonine kinase with a predominantly nuclear localization [1, 2]
To examine the impact of VRK1 overexpression on the properties of mammary epithelial cells, we chose to focus on MCF10a cells because they are non-transformed, maintain a near-diploid karyotype and have the ability to form polarized acini in 3D culture
The WT and kinase-dead proteins were expressed at ~20- and 12-fold higher levels than endogenous VRK1, respectively (Fig 1A); it is important to note that VRK1 overexpression does not affect the apparent doubling time of MCF10a cells in 2D culture [25.9 +/- 0.5h vs. 24.6 +/-1.5h (VRK1-overexpressing) and [5]]
Summary
Vaccinia-related kinase 1 (VRK1) is a serine/threonine kinase with a predominantly nuclear localization [1, 2]. It is highly expressed in proliferative tissues, tumors and cancer-derived cell lines [2,3,4,5,6,7,8,9]. VRK1 has been proposed to play a role in cancer progression by promoting the G1 to S cell cycle transition, in part by phosphorylating the CREB1 transcription factor and increasing Cyclin D1 mRNA levels [10, 11]. Other identified substrates of VRK1 include stress-responsive transcription factors such as p53 [15, 16], cJun [17], and ATF2 [18]
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